I've searched for the thread in which I learned of the existence of these products:http://focusingscreen.com/
but I couldn't find it. I took the chance and ordered one (Ec-A). I installed it today. It didn't work for me, so I put the original screen back in.

ClausC wrote:
Why didn't it work for you ?
Well, not to be pedantic, but I'm guessing you're really asking what was amiss, not why was it amiss. The whole frame was blurry; turning the focus ring had a visible effect in the center circle, but it never became clear; it was totally unusable. Quite a disappointment.

saneproduction wrote:
I am also curious, would love to install a super precision matte screen.
Yes, I think that would be good if it worked... but now I feel like Mark Twain's "cat who sat down on a hot stove lid."

Does the MkIII have setting adjustments that would allow you to change the way the camera meters to adjust for the focus screen that you have installed? I know the MkII and 6D do but I heard the MkIII didnt have any such setting. Seems odd that Canon left that option out on the MkIII

Ernie Aubert wrote:
Well, not to be pedantic, but I'm guessing you're really asking what was amiss, not why was it amiss. The whole frame was blurry; turning the focus ring had a visible effect in the center circle, but it never became clear; it was totally unusable. Quite a disappointment.

Sounds like you didnt install it properly, it only takes a minute mistake to put it out of focus.

Ernie Aubert wrote:
Well, not to be pedantic, but I'm guessing you're really asking what was amiss, not why was it amiss. The whole frame was blurry; turning the focus ring had a visible effect in the center circle, but it never became clear; it was totally unusable. Quite a disappointment.

A few suggestions:

Are you sure it is orientated correctly ? (it should be the polished/shiny side downwards) Since the 5DIII doesn't have (at least officially) replaceable focus screens, it appears Canon have removed the 'handling tab' which usually gave a clue about up and down.

Luta13 wrote:
Does the MkIII have setting adjustments that would allow you to change the way the camera meters to adjust for the focus screen that you have installed? I know the MkII and 6D do but I heard the MkIII didnt have any such setting. Seems odd that Canon left that option out on the MkIII

Ernie Aubert wrote:
There's no ambiguity in the way it's oriented. There's no menu item for screens in the 5D III. The instructions made no mention of "shims".

In some previous cameras there were shims that were used for spacing between the focusing screen. My 20D had them. I have never seen one in any camera that had an user interchangable focusing screens (ex 1 series, 5D MK2, ...), but since the 5DMK3's screen is not designed to be user interchangable who knows. However, I am sure that Roger would have mentioned the shims if he had run across them.